Even taking out a small area in today's modern combat fighters would do serious damage, and travelling at the speed of light there isn't much that maneuverability or current defensive countermeasure can do to deter a laser.
Perhaps the catch being that if you have LOS to half the planet then half the planet has LOS to you and presumably its a lot easier to build and operate a high energy laser on the ground rather than in high orbit.
That said, I'm thinking pumped-laser-tipped missiles could be very effective as e.g. anti-ship weapons. You could launch one straight up from your territory, and have it snipe the enemy ship from far away. The enemy would have to monitor a large portion of the sky for relatively small objects and react instantly with lasers of their own to counter that.
EDIT: I suspect these could be made into pumped-laser shells; good luck countering one that's launched from beyond the horizon.
Water is also a good RF screen, so such shield would also effectively blind and cripple the ship. Even discounting the energy use, no way they could keep that up continuously. Ramp-up time is probably large enough too (on the order of seconds), making surprise attacks very feasible. Not an ideal countermeasure, though I'm not sure what would be, beyond packing more ablative armor.